In the conventional compact disc (CD), the sampling frequency fs is about 44.1 kHz and the PCM method is used to record one sample as 16-bit digital audio data for each channel. There has been proposed a so-called super audio compact disc (SACD) in which a 1-bit audio stream data is recorded with a very high sampling frequency generated by the DSD (direct stream digital) method (for example, a frequency 64 times higher than the sampling frequency fs in the ordinary CD).
By oversampling delta-sigma modulation of an input signal with 64 fs, there is generated a 1-bit audio digital signal. Hereupon, the 1-bit audio digital signal is decimated into a multi-bit PCM code in the CD using the PCM method. In the SACD using the DSD method, however, the 1-bit audio digital signal is recorded directly to the SACD.
The frequency band of the 1-bit audio signal recorded to the SACD is about 100 kHz. This is rather wider than the frequency band of the signal in the PCM method adopted in the CD.
In playback of the SACD having recorded therein the same audio signal in such a wide frequency band as in the CD, sound components of relatively low frequencies can be reproduced to be audible to the human ears. For example, even when the SACD is played back in a player having no ample acoustic-pressure frequency response, such as a home audio player, the sound recorded in the SACD can be reproduced to be audible to the human ears. However, the human ears can hardly hear sound components of high frequencies, such as 20 kHz or higher, reproduced from the SACD. When an audio signal in a wide band including high frequencies is reproduced from the SACD, sound components thereof in the low frequency band are audible to the human ears but sound components in the high frequency band are not audible to the human ears.
When reproducing an audio signal from a sound source including up to sound components of high frequencies such as about 100 kHz and which has an acoustic level of sound components in a high frequency band not audible to the human ears is higher than that of sound components in a low frequency band, raising the reproduction level of an audio player used for the audio signal reproduction, with the ability of the human ears taken in consideration, for the sound components in the low frequency band to be audible to the human ears at a sufficiently high acoustic pressure level, will result in an elevated acoustic pressure level of the sound components in the high frequency band for the high reproduction level. Since the human ears cannot hear any sound components in the high frequency; band, the reproduction level may possibly be raised above the capability of the audio player with the result that the power amplifier and speaker of the audio player is likely to be greatly loaded and damaged as the case may be.
Therefore, a function to monitor the sound level of sound components difficult to capture by the human ears should be provided in a player designed to reproduce audio signals in a wide band including from a low frequency to a high frequency such as 100 kHz, such as audio signals recorded to the SACD, or in an recorder to record such audio signals and an apparatus to produce a medium such as a compact disc or the like.
On the other hand, different from the multi-bit digital signal obtained by the conventional PCM method, the 1-bit digital signal obtained by the delta-sigma modulation includes also frequency components beyond the audio band of frequencies audible to the human ears. So, the level of the 1-bit digital signal cannot be known if left as it is. Conventionally, to know the level of a signal, only a necessary frequency component of the signal are extracted using a digital filter and the level of the signal is indicated based on the data thus extracted.
When the digital filter is used, however, different outputs may possibly result from the same input signal depending upon how the digital filter has been designed, namely, depending upon the type, factor and computing word length thereof. For example, since it is difficult to determine the level of the signal uniquely, indication of the same signal supplied to a player will possibly vary from one apparatus to another. Since the digital filter uses a multiplier and adder, its circuit is likely to be complicated and large in scale, which will cause a large manufacturing cost and increased power consumption of the circuit.
Also, if the digital filter is strictly designed for the completely same output from an input, it will be less flexible in performance and manufacturing cost so that the product design will be difficult in practice.